Dehiscence (botany)

Manipulation of dehiscence can improve crop yield since a trait that causes seed dispersal is a disadvantage for farmers whose goal is to collect the seed. Many of the agronomically important plants have been bred for reduced shattering.

Dehiscence occurs through breakage of various parts of the enclosing structure; the mechanisms can be classified in various ways, but intermediate forms also occur.

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  Transverse dehiscence of a pair of anthers
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  Longitudinal dehiscence of a pair of anthers
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  Valvular dehiscence of a pair of anthers
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  Poricidal dehiscence of a pair of anthers

Anther dehiscence is the final function of the anther that causes the release of pollen grains. This process is coordinated precisely with pollen differentiation, floral development, and flower opening.

The anther wall breaks at a specific site. Usually this site is observed as an indentation between the locules of each theca and runs the length of the anther, but in species with poricidal anther dehiscence it is instead a small pore. If the pollen is released from the anther through a split on the outer side (relative to the center of the flower), this is extrorse dehiscence, and if the pollen is released from the inner side, this is introrse dehiscence. If the pollen is released through a split that is positioned to the side, towards other anthers, rather than towards the inside or outside of the flower, this is latrorse dehiscence.

The stomium is the region of the anther where dehiscence occurs. The degeneration of the stomium and septum cells is part of a developmentally timed cell-death program. Expansion of the endothecial layer and subsequent drying are also required for dehiscence. The endothecium tissue is responsible for the tensions that lead to splitting of the anther. This tissue is usually one to several layers thick, with cells walls of uneven thickness due to uneven lignification. The cells lose water, and the uneven thickness causes the thinner walls of the cells to stretch to a greater extent. This creates a tension that eventually leads to the anther being split along its line of weakness and releasing pollen grains to the atmosphere.

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  Before/During images of anther dehiscence in the common Milk Pea
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  Poricidal anther dehiscence
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  Longitudinal latrorse anther dehiscence

Flower buds of Eucalyptus and related genera open with circumscissile dehiscence. A small cap separates from the remainder of the bud along a circular horizontal zone.

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  Corymbia ficifolia
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  Eucalyptus globulus

There are many different types of fruit dehiscence, that involve different types of structures. Some fruits are indehiscent, and do not open to disperse the seeds. Xerochasy is dehiscence that occurs upon drying, and hygrochasy is dehiscence that occurs upon wetting, the fruit being hygroscopic. Dehiscent fruits that are derived from one carpel are follicles or legumes, and those derived from multiple carpels are capsules or siliques.[3]

One example of a dehiscent fruit is the silique. This fruit develops from a gynoecium composed of two fused carpels,[3] which, upon fertilization, grow to become a silique that contains the developing seeds. After seed maturation, dehiscence takes place, and valves detach from the central septum freeing the seeds. This is also known as shattering and can be important as a seed dispersal mechanism. This process is similar to anther dehiscence and the region that breaks (dehiscence zone) runs the entire length of the fruit between the valves (the outer walls of the ovary) and the replum (the persisting septa of the ovary). At maturity, the dehiscence zone is effectively a non-lignified layer between two regions of lignified cells in the valve and the replum. Shattering occurs due to the combination of cell wall loosening in the dehiscence zone and the tensions established by the differential hygro-responsive properties of the drying cells.[4]

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  Poppy fruit showing poricidal dehiscence; the seeds exit through pores beneath the "crown"
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  Peanuts: an indehiscent subterranean legume fruit
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  Thlaspi arvense, with fruit that are dehiscent siliques
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  Rhododendron capsules have septicidal dehiscence; the fruit splits through the septa between the carpels
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  Iridaceae capsules have loculicidal dehiscence; the fruit splits through the ovary wall of each carpel, allowing the seeds to exit directly from the locule
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  Anagallis fruits open with circumscissile dehiscence. A small cap separates from the remainder of the fruit along a circular horizontal zone.
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  Anagallis fruit, circumscissile dehiscence
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  Spathoglottis plicata capsules, like in most orchids, split longitudinally along three to six slits while remaining closed at both ends

Endothecium tissue found in moss capsules functions in a similar way in dehiscence to the endothecium in the walls of anthers (see above).

Many leptosporangiate ferns have an annulus around the sporangium, which ejects the spores. Eusporangiate ferns do not generally have specialized dehiscence mechanisms.

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  Sporangium dehiscence through a horizontal slit in Botrychium, a eusporangiate fern.

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  Various sporangia of myxomycetes that dehisce in varied ways

• Abscission—separation of structures that leads to their loss
• Anthesis—the opening of flowers
• Elaters—structures that form inside a sporangium and aid in spore dispersal of horsetails, liverworts, and hornworts
• Loment—a type of fruit that breaks apart but is not dehiscent
• Schizocarp—a type of fruit that breaks apart and may or may not be dehiscent.